feat(#363): add descriptive text to loading spinners (#406)

* feat(#363): add descriptive text to loading spinners

- Updates LoadingState composable to accept optional subtitle
- Shows 'Fetching model providers...' on ModelScreen
- Shows 'Loading channels...' on ChannelsScreen
- Removes dead/unreachable CircularProgressIndicator from ModelScreen

* fix(#363): pass loading_state_subtitle_models string to LoadingState

---------

Co-authored-by: M57 <hy4ri@users.noreply.github.com>

app/src/main/java/com/m57/hermescontrol/ui/channels/ChannelsScreen.kt

@@ -73,7 +73,10 @@ fun ChannelsScreen(
     ) { paddingValues ->
         when {
             state.isLoading && state.platforms.isEmpty() -> {
-                LoadingState(modifier = Modifier.padding(paddingValues))
+                LoadingState(
+                    subtitle = stringResource(R.string.loading_state_subtitle_channels),
+                    modifier = Modifier.padding(paddingValues),
+                )
             }
 
             state.errorMessage != null -> {